The present relates to turbochargers for internal combustion engines and more particularly to a simplified assembly of and arrangement for lubricating the bearing system of a turbocharger.
Turbochargers are widely used on internal combustion engines, and in the past have been particularly used with large diesel engines, especially for highway trucks and marine applications. In distinction to superchargers, which derive their power directly from the crankshaft of the engine, turbochargers are driven by the engine exhaust gases. Exhaust gases are directed to and drive a turbine, and the turbine shaft is connected to and drives the compressor. Ambient air is compressed by the compressor and fed into the intake manifold of the engine.
More recently, in addition to use in connection with large diesel engines, turbochargers have become popular for use in connection with smaller, passenger car power plants. The use of a turbocharger in passenger car applications permits selection of a power plant that develops the same amount of horsepower from a smaller, lower mass engine. Using a lower mass engine has the desired effect of decreasing the overall weight of the car, increasing sporty performance, and enhancing fuel economy. Moreover, use of a turbocharger permits more complete combustion of the fuel delivered to the engine, thereby reducing the hydrocarbon emissions of the engine which contributes to the highly desirable goal of a cleaner environment.
As the use of turbochargers finds greater acceptance in passenger car applications, three design criteria have moved to the forefront. First, the market is demanding that all components of the power plant of a passenger car, including the turbocharger, must provide reliable operation for a much longer period than was demanded in the past. That is, while it may have been acceptable in the past to require a major engine overhaul after 80,000-100,000 miles, it is now necessary to design engine components for reliable operation in excess of 200,000 miles of operation. This means that extra care must be taken to ensure proper lubrication of bearings supporting devices that rotate at very high rotational speeds, as in a turbocharger.
The second design criterion that has moved to the forefront in passenger car applications is that the power plant must meet or exceed very strict requirements in the area of minimized hydrocarbon emissions. Third, with the mass production of turbochargers for smaller passenger cars, it is highly desirable to design a turbocharger that meets the above criteria and is comprised of a minimum number of parts, which parts are easy to manufacture and easy to assemble, in order to provide a cost effective and reliable turbocharger.
As stated above, the demand for engine components that provide an extended service life requires that extra care must be taken to ensure proper lubrication of bearings that support devices rotating at very high rotational speeds, as in a turbocharger. In the prior art, two basic systems have been adopted to deliver lubricating oil to the critical wear points of a turbocharger using two floating journal bearings. First, the central bearing housing can be provided with lubricating oil channels directed to the top of the journal bearings, the so-called xe2x80x9ctop-deliveredxe2x80x9d system. With this system, oil is delivered to the top of the journal bearings, usually at the axial center of the bearings, and the bearings are normally provided with radial apertures in the center of the bearing to allow flow of the lubricating oil radially inwardly to the interface between the shaft and the inside diameter of the journal bearing. In this system, the oil must be supplied at high pressure in order to ensure that it will migrate inwardly through an aperture in a journal bearing while the journal bearing is spinning at a very high rate.
The second basic system for delivering lubricating oil to the journal bearings of a turbocharger is to deliver the oil to the center of the rotating shaft and allow the oil to migrate axially outwardly along the shaft and over the bearings before being released to the oil return sump and to the engine crankcase. In both of these systems, in order to provide adequate lubrication to the bearings, a high flow rate of oil has been provided to ensure that adequate coverage of the bearing surfaces is obtained. Especially in connection with the top-delivered system, a high percentage of the oil flowing through the system contacts only the outboard half of the outside diameter of the journal bearing before being expelled into the oil return sump. This means that a very high volume of flow must be provided to obtain any oil film coverage of the other surfaces of the journal bearing.
This high flow of oil through the bearing housing of a turbocharger increases the opportunity for oil to leak from the bearing housing into the turbine or compressor portions of the turbocharger. Internal combustion engines, whether diesel or gasoline, are designed for optimum combustion of the fuel for which they are designed. In either type of engine, when engine crankcase lubricating oil is introduced into the combustion chamber of the engine, it is not burned effectively, and a large portion of that oil is emitted as an undesired hydrocarbon pollutant. Engine manufacturers have been diligent in reducing the amount of lubricating oil that is allowed to enter the combustion chamber of the engine by improving piston ring and valve stem seal designs, and the like. Unfortunately, turbocharger design has not kept pace with this trend.
As stated, turbochargers commonly use crankcase oil to lubricate the rotating bearing interfaces as well as the thrust surfaces that limit axial excursions of the shaft and its turbine and compressor wheels. Since turbochargers operate at extremely high rotational speeds, sometimes in excess of 200,000 RPM, generous lubrication of these bearing surfaces is critical in order to provide a turbocharger capable of a long and reliable service life. With this high flow rate of oil over the journal bearings comes the possibility that some percentage of the oil will escape past the barriers set up in the turbocharger to prevent lubricating oil from entering either the turbine housing or the compressor housing.
More specifically, if lubricating oil from the center bearing housing migrates beyond the piston ring seal provided to prevent such migration at the turbine end of the housing, lubricating oil will enter the turbine housing and will be expelled with the exhaust flow out of the engine into the atmosphere. On the other hand, if lubricating oil from the center bearing housing migrates beyond the piston ring seal at the compressor end of the housing, the lubricating oil will enter the compressor housing and will be injected into the combustion chamber of the engine where it will not be properly burned and will be emitted by the engine as an undesired hydrocarbon pollutant. Unfortunately, as a result of this phenomenon, it is commonly believed that over half of the hydrocarbon emissions of turbocharged engines come from oil leakage through the turbocharger, not from the engine itself.
Thus, it seems that these two design criteria point a designer in different directions. That is, if it is desired to achieve longer service life of a turbocharger, the flow of oil over the bearings should be increased to minimize metal-to-metal contact between parts and decrease wear of the parts. On the other hand, if hydrocarbon emissions of the engine are to be decreased, oil flow through the bearing housing should be minimized to decrease the opportunity for oil leakage into the turbine or compressor housings of the turbocharger.
Many attempts have been made to minimize leakage of oil from a turbocharger bearing housing, but these have always taken the form of adding a number of parts or a new sub-assembly, such as an oil deflector, extra seals, or the like. While this may assist in reducing oil leakage from the bearing housing, it is contrary to the third design criterion mentioned above. That is, adding more parts or a new sub-assembly tends to make the turbocharger more complicated and expensive, when it is desired to make the turbocharger simpler and easier to manufacture and assemble.
An earlier attempt at providing a simplified bearing system is shown in U.S. Pat. No. 3,993,370 to Woollenweber. That patent shows a bearing system in which the journal bearings are constrained to ride on the bearing lands of the center housing by a shoulder of the center housing on the inboard side of the bearings, and by a shoulder formed on the shaft at the turbine end and a thrust collar that rotates with the shaft at the compressor end of the center housing. With this arrangement, none of the thrust-bearing surfaces is between the shaft, which rotates at very high speed, and a stationary surface. Rather, the thrust-bearing surfaces are between the end surfaces of the journal bearings, which rotate at speeds less than the speed of the shaft, and either a stationary surface on the housing or a shoulder or collar carried by the shaft. Thus, the bearing assembly of Woollenweber provides reduced relative speed between the rotating assembly of the turbocharger and the thrust-bearing surfaces on the combined journal and thrust bearings, regardless of the direction in which the thrust force acts, and the conventional thrust bearing assembly has been eliminated.
Lubrication is still provided in the conventional manner of a positive pressure, top-delivered system that requires oil flow radially inwardly through a spinning bearing in order to deliver lubricating oil to the rotating interface between the shaft and the inside diameter of the journal bearing. A radial aperture through the journal bearing is provided for that purpose.
Another simplified bearing system is shown in Swiss Patent No. 407,665 to Buechi. That patent shows, in the context of a turbocharger, a pair of floating bearings constrained to float at their respective bearing lands within a center housing (See FIGS. 1 and 2). On the outboard side, the bearings are constrained from axial movement by a collar that is carried by the shaft. On the inboard side, the bearings are constrained by axial abutment surfaces on a pair of rings, which in turn are held in their axial position by conventional snap rings.
Oil is delivered centrally between the bearings and is allowed to migrate axially outwardly along the shaft to reach the bearings. It is not clear whether any attempt is made to balance the oil pressure on various surfaces of the bearings or to control the flow of oil over those surfaces to achieve maximum efficiency of lubrication of the bearings with minimum flow rate of oil. It is clear that no axial aperture is provided through the bearing, nor is any axial groove provided in the inside diameter or the outside diameter of the bearing to provide a means for controlling the flow rate of oil across the journal bearings. As a further indication that such control is not present, it is noted that Buechi believed two piston rings were required in each axial direction to control undesired oil flow into either the turbine or compressor housings.
It appears that if the Buechi structure employed normal bearing clearances between the journal bearing and the shaft, and between the journal bearing and the housing, the flow of oil around the journal bearing would be too low to provide adequate lubrication to the thrust surfaces at the axial ends of the journal bearings. On the other hand, if the clearances between the journal bearing and the shaft, and between the journal bearing and the housing, were large enough to provide oil flow adequate to lubricate the thrust bearings, the journal bearings would not provide stable rotational support for the shaft and the turbine and compressor wheels.
It is therefore a primary object of the invention to provide a turbocharger bearing system characterized by a highly efficient, controlled lubrication system that permits excellent lubrication of the bearings with a minimum of oil flow through the bearing housing, thereby providing a turbocharger that is reliable and durable in operation.
Another object of the invention is to provide a turbocharger bearing assembly that will significantly reduce the amount of oil that is leaked into the engine intake or exhaust streams, thereby greatly reducing the hydrocarbon emissions of the engine.
A further object of the invention is to accomplish the above objects while providing a turbocharger bearing assembly that is greatly simplified, being comprised of a reduced number of parts, each of which is easy to manufacture, and which are easy to assemble to form a turbocharger that is efficient and durable in operation.
In accordance with the present invention, these and other objects are achieved by providing a greatly simplified turbocharger assembly that allows accurate and efficient control of oil flow over the bearings, thereby permitting excellent lubrication of the bearings with a reduced amount of oil flow through the bearing housing, resulting in significantly lower hydrocarbon leakage from the turbocharger into the engine or engine exhaust, and ultimately lower hydrocarbon emissions by the engine. In a preferred embodiment of the invention, this is accomplished by receiving a turbocharger shaft in a central bore in the bearing housing and supporting that shaft on a pair of floating journal bearings riding on bearing lands formed in the housing. The journal bearings are axially constrained on their inboard sides by a shoulder in the bearing bore of the housing and on their outboard side by a step in the shaft at the turbine end of the bearing housing and by a flinger sleeve carried by the shaft at the compressor end of the bearing housing. The flinger sleeve is held in place on the shaft by the compressor wheel, which in turn is threaded onto the shaft by cooperating threads at the nose of the compressor wheel, thereby eliminating the need for a washer and/or nut for attaching the wheel.
To provide lubrication to the bearings, a central lubrication inlet port is provided in the housing in communication with the central bore between the bearing lands. Lubricant proceeds from the inlet port axially in both directions along the shaft through a gap between the shaft and the central bore to the journal bearings. At the journal bearings, the lubricant flows between the shaft and the journal bearings, past the axial ends of the journal bearings to lubricate the axial thrust surfaces formed thereon, and over the journal bearings to lubricate the rotating interface between the journal bearings and the housing.
In order to control oil flow through the housing and balance oil pressure around the journal bearings, great care is taken to size the various passages through the housing and around the bearings, and an axial passage is provided in the bearings extending from the inboard end of the bearings to the outboard ends thereof. This axial passage can take the form of a bore through the bearing, or a groove in the inner and/or outer diameter of the bearings.
More specifically, the passages begin with the inlet port that has a first cross sectional area. The oil then proceeds to an axial channel defined by the difference between the area of the central housing bore and the cross sectional area of the shaft disposed in the bore. This difference must be multiplied by two since the oil flow proceeds in two axial directions. If the size of the bore or the shaft is different in the two axial directions, this must be taken into account in determining the total cross sectional area of the combined axial channels. It is also possible that these axial channels may not have a constant cross sectional area along their axial extent. That is, either the central bore in the housing or the portion of the shaft disposed in the central bore could be tapered causing the cross sectional area to change along the axial length of the channel. In this instance, the area under consideration that determines oil flow is the smallest cross sectional area in that channel.
The third area to be controlled to achieve the desired flow of oil across the journal bearings is the total area available for oil flow over, under, around and through the journal bearings in the bearing lands of the housing. This area can be defined as the total area of the bearing lands minus the cross sectional area of the shaft disposed in the bearing lands, minus the cross sectional area of the journal bearings, which does not include the area of any aperture or groove in the journal bearing. Again, the sum in both axial directions must be considered to determine the total area of the third channel.
In general, the areas must be arranged so that the first area is equal to or greater than the area of the third channel around the journal bearings. In order to achieve an oil flow rate adequate to lubricate the axial end thrust surfaces of the journal bearings, an axial passage must be provided in the journal bearings, either under, over, or through the journal bearings. As a result, the flow area through the third channel will necessarily be greater than the flow area over a journal bearing located in the third channel with no axial apertures or grooves provided for this purpose. Since the journal bearings of the present invention act as both rotational journal bearings and as thrust bearings, it is important that all faces, both radial and axial, receive adequate lubrication. This is accomplished by metering the flow rate of oil across the bearings by use of an aperture or groove in the journal bearings, thereby providing a flow rate that is greater than would have been available if no such aperture or grooves were included, and less than the wasteful and polluting quantities of oil emitted into the atmosphere when a top-delivered system of lubrication is used. With this arrangement, an adequate flow rate of oil can be maintained to keep to keep the inside and outside diameter portions of the journal bearings properly lubricated, and to provide adequate lubrication to the axial thrust surfaces of the journal bearings, while still keeping the total flow rate of oil to a minimum to reduce the potential for oil leaking to the turbine or compressor housings and thus creating unwanted hydrocarbon emissions.
Preferably, the channels proceeding in the two axial directions from the inlet port are symmetrical for ease of manufacture of the parts and for uniform control of oil flow in the two directions. In addition, the journal bearings themselves are preferably identical. Alternatively, it may be desirable to provide a greater flow rate of oil in one direction than in the other. For example, it may be desirable to provide a greater flow of oil over the journal bearing at the turbine end of the bearing housing than that provided at the compressor end because the turbine end is hotter and the journal bearing at that end needs more cooling effect from the oil flow. To accomplish this, the cross sectional areas of the axial channels leading to the journal bearings can be adjusted to promote this uneven flow. Further, the cross sectional area of the oil flow channels in the region of the bearing lands can be modified so that an increased flow is permitted over the journal bearing at the turbine end of the bearing housing as compared to the flow over the journal bearing at the compressor end of the bearing housing. In a preferred embodiment of the invention, the cross sectional area of the combined, axial channels is less than the area of the inlet port and greater than the cross sectional area of the third channel around the journal bearings.
To promote axial flow of lubricating oil away from the journal bearing at the turbine end of the bearing housing, the shoulder of the shaft is provided with an abutment surface to abut the outboard end of the journal bearing, and that abutment surface has an outer diameter substantially less than the outer diameter of the journal bearing. To direct oil flow from the journal bearing into the oil return sump and away from the turbine housing, the shoulder has an increased diameter portion axially spaced from the journal bearing for flinging oil off of the shaft before it can migrate toward the turbine end of the shaft. Similarly, a flinger sleeve at the compressor end of the bearing housing has an abutment surface to abut the outboard end of the journal bearing at the compressor end of the housing, and that abutment surface has an outer diameter substantially less than the outer diameter of the journal bearing. Also, the flinger sleeve has an increased diameter portion axially spaced from the journal bearing for flinging oil off of the shaft before it can migrate toward the compressor end of the shaft.
In a preferred embodiment of the invention, the journal bearings are provided with chamfers at the intersection of the inner surface and the outer surface with the inboard and outboard ends of the bearings. In order to promote flow of lubricating oil to the interface between the shaft and the journal bearing, the chamfer at the intersection of the inner surface of the bearing with the inboard and outboard ends is greater than the chamfer at the intersection of the outer surface with the inboard and outboard ends.
Since the journal bearings of the present invention serve both as journal bearings for rotationally supporting the turbocharger shaft and as thrust bearings for limiting axial excursions of the shaft, the axial end faces of the journal bearings are preferably provided with radial grooves to promote flow of oil across those thrust surfaces. To permit the journal bearing end faces to serve as effective thrust bearing surfaces, the radial grooves preferably have adjacent ramp portions leading to flat thrust bearing lands on the end faces. In the most preferred embodiment of these bearings, the radial grooves have adjacent ramp portions leading to flat thrust bearing lands on both sides of said grooves forming symmetrical thrust surfaces on said inboard and outboard ends. With this arrangement, the journal bearings can themselves be symmetrical so that they can be assembled in the turbocharger assembly in either axial direction, thereby simplifying the assembly step and making it easier to manage.
In summary, concerning the above-described structure, it is noted that complicated turbochargers in the prior art often are constructed of more than one hundred parts. Even relatively simple turbochargers commonly used today are comprised of forty parts or more. Remarkably, the turbocharger of the present invention provides a fully functional turbocharger that is efficient and durable in operation and is comprised of a total of twelve parts.
Lastly, a method for lubricating a rotating shaft is disclosed that employs a turbocharger structure as set forth above and includes the steps of supplying the bearing housing with a lubricant under pressure, and channeling that lubricant through a series of lubricant transmission channels, the last such channel having a cross sectional area equal to or smaller than the first channel. Preferably, each successive channel has a cross sectional area equal to or smaller than the preceding channel. The method further includes the step of balancing the pressure of the lubricant to achieve substantially equal pressure on all faces of the journal bearings in the bearing lands, and the step of forming a lubricant film on the end thrust surfaces of the journal bearings.
In all of the above-described embodiments of the invention, the apparatus employed to practice this invention is relatively easy to manufacture and has a minimum number of parts. In addition, the lubrication system is efficient and effective, thereby producing a turbocharger capable of a very long useful life while still reducing the amount of lubricant necessary to achieve these ends, and, therefore, significantly reducing the amount of hydrocarbon emissions caused by the turbocharger.
These and other aspects of the invention will be more apparent from the following description of the preferred embodiments thereof when considered in connection with the accompanying drawings and appended claims.